REQ: Help - My machine has stopped running.

I have an AMD ATX system that has quit running.

I was restoring its 13G HD to what it was before pretty much. I
completely erased it. I had it 99% finished, and all was running just
fine. I even did a scandisk and defrag on it. I turned the system
off, and cut my grass. When I came back, I turned the machine back
on, and found that it wouldn't even start.
As I watched it, I saw that the CPU fan would start for an instant,
then stop. Indicative of a short I am thinking.

I tried a few things - (1) I disconnected the two PCI cards (modem and
video) - same result; (2) I then disconnected ALL the drives (13G HD,
CDR, Backup Tape Drive, Floppy) - same result; (3) I removed the 256M
SDRAM - same result; (4) I then replaced the power supply with another
one - same result; (5) I then tried different wall power receptacles -
same result. All this pretty well leads me to believe that either the
MOBO or CPU has crapped out (or both).

I don't know what else to try. I didn't do anything to cause the
problem that I know about.

Anyone have a suggestion?

Thanks